1. Field of Invention
The present invention relates to the field of optical fibers and particularly to new compositions for glass optical waveguide fibers which can provide fibers with controlled internal stress characteristics.
2. Description of the Prior Art
At present most if not all of the low loss (less than 20 db/km) optical waveguide fiber being installed for telecommunications applications is glass core-glass clad fiber made of fused silica and doped silica materials. U.S. Pat. No. 3,659,915 describes glass optical fibers made of such materials, and identifies a number of dopant materials, including TiO.sub.2 and Al.sub.2 O.sub.3, which can be added to fused silica to raise its index of refraction.
The bulk of the optical fiber sold for commercial use contains GeO.sub.2 as a dopant for increasing the refractive index. GeO.sub.2 is effective in raising the refractive index of silica, does not introduce significant optical absorption at wavelengths where the fibers are to be used, and is easily derived from the commercially available volatile chloride GeCl.sub.4.
The need for glass stress control in optical fiber design and manufacture is always of some concern but is particularly important in the manufacture of optical fibers of high numerical aperture (NA). The numerical aperture, which for an optical fiber is a number representative of its light-gathering power, depends on the difference between the refractive index of the core of the fiber (n.sub.1) and the refractive index of the cladding (n.sub.2), as follows. ##EQU1##
In general, the refractive index of fused silica is raised only to an extent proportional to the amount of index-increasing dopant added to the silica; thus large amounts of core dopant relative to cladding doping levels are required to obtain the high index core needed for high numerical aperture fibers. The amount of dopant which can be added to the core of an optical fiber is limited, however, since all dopants currently used for optical fiber fabrication can undesirably change properties other than refractive index, such as viscosity and thermal expansion, as they are added to the glass.
Thermal expansion changes can be undesirable since they cause an expansion mismatch between the cladding and the core which can introduce large stresses into the optical fiber or into the glass preform from which the optical fiber is made. Thus it is difficult to produce a high numerical aperture fiber using, for example, GeO.sub.2 as a core dopant because of the large amount of GeO.sub.2 which must be added to the core glass. To achieve a numerical aperture greater than about 0.3, more than 20 mole percent of GeO.sub.2 must be introduced into the core. This results in a core glass with an average linear coefficient of thermal expansion which is more than 23.times.10.sup.-7 /.degree.C. greater than the expansion of the pure fused silica cladding glass. Depending upon the manufacturing process used, such stresses can cause considerable difficulty, particularly with respect to stress cracking of the glass preform.
The problem of preform stress has been recognized and a number of approaches to minimize the problem have been proposed. U.S. Pat. No. 4,194,807 proposes the use of a silica-free core glass composition based on GeO.sub.2 in combination with a silicate glass cladding material of relatively high expansion coefficient. U.S. Pat. No. 4,038,090 discloses multicomponent silicate glass compositions from which fiber optics with low internal stress can be made by ion-exchange processing.
U.S. Pat. No. 4,177,319 describes alkali-MgO-SiO.sub.2 glass compositions, optionally containing Al.sub.2 O.sub.3, which can produce fiber optics with numerical apertures up to or slightly exceeding 0.3 by ion-exchange processes. However, multicomponent glasses such as described in that patent are typically produced by melting and do not exhibit the low optical attenuation characteristics of the doped silica glasses produced by vapor phase deposition. The same is true of multicomponent glasses such as described in U.S. Pat. No. 4,264,131, which are proposed as optical fiber glasses of high chemical durability. British Pat. No. 1,160,535 discloses optical fiber compositions containing Al.sub.2 O.sub.3, SiO.sub.2 and at least two alkaline earth metal oxides from the group MgO, CaO and BaO; again these glasses are produced by melting and at least four components are needed.
For vapor phase glass manufacture it is particularly desirable from the standpoint of equipment design and composition control that the number of glass forming components by minimized. Hence ternary or simpler compositions are to be preferred over quaternary or more complex glasses. Also, present optical fiber designs require a high degree of control over composition profile. This makes readily diffusible glass components such as the alkali metals unattractive, and glasses essentially free of alkali metal oxides are used to fabricate commercial low-loss optical fibers.
In addition to fibers of high numerical aperture, another area where fiber stress characteristics are of concern is in the design of polarization-retaining simple-mode optical waveguides. U.S. Pat. No. 4,395,270 discloses examples of such waveguides wherein polarization retention is effected by a stress-induced birefringence within the fiber.
The structure of polarization-retaining fibers such as described in this patent typically includes a pair of diametrically opposed longitudinal stress elements which are regions of glass running parallel with and on opposite sides of the fiber core. These glass stress elements have a thermal expansion coefficient differing from that of the fiber cladding in which they are disposed, with the degree of difference determining the amount of stress present.
A particularly desirable design for such a fiber would be one in which the stress elements differ in thermal expansion but not in refractive index from the adjacent cladding glass. Maintaining a close or identical refractive index relationship between a stress element and any adjacent cladding material would minimize the propagation of unwanted cladding modes by the optical fiber.